Does Blender support dual-GPU rendering if the two GPUs aren't the same?
You can, as long they are from the same brand (AMD, or NVidia) you should be able to use them simultaneously to render in Blender Cycles.
If you keep them out of any proprietary GPU pairing technologies setup like SLI or Crossfire, the operating system detects them as two discrete GPUs and see both graphics cards, Blender should be able to see them as discreet GPUs in the user preferences.
This will virtually decrease render times linearly, proportionally to each additional compute device performance, as opposed to pairing technologies which have significant performance penalties and lose efficiency with each additional GPU unit.
You can then prior to rendering select which one to use, or both simultaneously, if you want. You can also have each instance of Blender use its own GPU independently, although you will likely experience performance penalties caused by bottlenecks anywhere else in your system, namely available memory, and CPU resources for pre-processing your scene.
Under User Preferences &gt; System &gt; Cycles Render Device you can activate CUDA processing and pick from the list of available devices below to chose which ones to use for each running session. From the Properties Window &gt; Render Properties you can then set the device to GPU while Cycles is set as active render engine.
Have in mind that when using multiple GPUs for rendering a single scene you will always be bound by the available memory on the card with the least VRAM amount. Since GPUs still can't share memory access, a copy if the whole scene must be fully loaded into each individual graphics card memory, so it must completely fit into the available RAM of every single GPU, failing any of them will abort rendering.
Having two GPUs from very far apart generations may eventually cause problems in the long term. As each generation ages it is eventually phased out, and becomes deprecated. At a certain point newer drivers stop supporting them and one last driver version compatible with the older model is launched, and you will essentially be stuck at that release if you wish to keep using your older GPU.
Even if newer drivers support your newest graphics card, since you can't have two different driver versions installed or running side by side, you still be forced to keep that older release while you plant to use the older generation. Some newer software (like games) may some times not work properly on older drivers, or require newer versions to even launch, forcing you to either update and lose use of the older card, or not update and miss out on the benefits of newer drivers.